Vibration in the form of very low amplitude, omnidirectional motion is naturally present in any building, and is present at varying levels at all frequencies up to very high acoustic frequencies. The acceleration associated with such vibrations introduces stress into the structures of many types of sensitive equipment and can deform those structures enough to degrade their performance. Soft pneumatic isolators can isolate such payloads from such environmental noise for frequencies as low as 1 Hz and above. These isolators provide isolation for all degrees of freedom of the payload (e.g. vertical, horizontal, tilt and twist). A payload supported on such pneumatic isolators will typically respond to multi-axis floor motions or forces directly applied to it by moving in many degrees of freedom. Such isolators must provide internal damping to limit the duration and amplitude of such response.
Another function of pneumatic isolators is to dampen the motion of the payload caused by movement of components supported by the payload. In the semiconductor industry, for example, it is common to have a heavy and fast moving stage (robot) carrying a wafer, which stops at different `sites` on the wafer to make an inspection. The stage motion causes the payload to move on its isolators. The measurement, however, cannot take place until this motion dies away. Thus it is critically important to the throughput of such systems that the isolators damp as quickly as possible.
The basic two-chamber pneumatic isolator, used for many years, is limited in how much damping it can provide. It is known to use external dampers of different types to enhance damping but the problem with this is the mounting, adjustment, and alignment of these external dampers. Some systems implement horizontal dampers but these dampers do nothing to improve the vertical damping of the isolators.
Exemplary of prior art, two chamber isolators is the Gimbal Pistons.RTM. isolator, Technical Manufacturing Corp., Peabody, Mass. As the payload moves up and down, air is forced through a small orifice between the two chambers. This damps the system, but only for the vertical motion of the isolator's piston. Moreover, the level of damping is limited by the finite ratio of the upper chamber's air volume to the lower chamber's air volume (the smaller this ratio the better). With this design the ratio of the isolator's spring to damping constant is approximately independent of the payload's mass. This gives a `Q`, or quality factor, which is nearly independent of changes in the payload (the settling time for the system does not change with changing payloads).
Horizontal damping in the system is less than the vertical damping and comes mainly from two sources: horizontal to tilt coupling, and an elastomer in the piston well. The first is a means for horizontal motion to be converted into tilt motion of the payload, which in turn `exercises` the isolators in the vertical direction. This happens when the center of mass of the payload is above the effective support points for the isolators (which is the usual case). The elastomer in the bottom of the piston well is also compressed with horizontal payload motions, and since it is not perfectly elastic, it damps the motion (just as some rubber materials will not bounce well when dropped on the floor).
An isolator such as described in U.S. Pat. No. 4,364,184 has previously been modified such that the inner chamber surrounding the support rod was half-filled with a 10,000 cst oil. This improved the damping, but could not be used to bring an isolator to critical damping.
The present invention uses an improved geometry of the just described isolator and a relatively viscous medium to achieve the high levels of force required to reach critical or near critical damping.
The present invention provides high levels of damping for both the vertical and horizontal motions of the payload. The level of the damping force (in terms of pounds of force for a given payload velocity) is fixed, however, and does not scale with the payload mass. This means, for example, that if a payload is doubled, the quality factor Q is also doubled. The preferred use for this type of isolator is for systems in which robotic motions are strongly driving the payload (such as in semiconductor manufacturing equipment). In such equipment the payload is usually fixed, and the damping can be adjusted to the desired level during the manufacture of the isolator by changing the viscosity of the viscous medium, the geometry of the tube/piston well combination, or simply changing the level of viscous medium in the isolator.
The present invention embodies a multi-axis isolator (two horizontal directions and vertical) and uses a viscous medium for damping which increases the level of damping force 2 to 10 times over that provided by the best prior art air-based dampers, for both vertical and horizontal motions. This level of damping can bring a system to `critical damping`.
Broadly, the invention comprises a pneumatic isolator having a housing which defines a pneumatic air chamber. Within the air chamber is a damping chamber defined by a chamber wall. A viscous medium is disposed in the damping chamber. A bob, having a longitudinal axis, is rigidly fixed to the payload with regard to its vertical motion and immersed in the viscous medium. In response to horizontal and/or vertical motion of the payload, the bob moves through the viscous medium and acts to dampen the payload motion in all translational motions.
In a preferred embodiment the bob comprises a gimbal piston head and a piston well having a bearing surface. A rod-like support member supports a payload, passes through the piston well and engages the bearing surface. The piston well extends into the damping chamber and into the viscous medium. The outer surface of the piston well is spaced apart from the opposed surface of the damping chamber and defines a gap therebetween. The viscous medium fills at least a portion of the gap. When the horizontal and/or vertical motion acts on the isolator, the piston well moves displacing the viscous medium in the gap resulting in a damping force which is generally proportional and opposite to the velocity of the piston well moving against the viscous medium.
Although the preferred embodiment will be described with the bob being exemplified by a piston assembly, as used in this disclosure, the term bob also includes isolators such as those vibration isolation mounts which have an internal pendulum assembly and piston assembly which serve the same function as the present invention's gimbal well, and could be similarly modified for improved damping. Also, the term bob includes those isolators which provide for vertical and horizontal isolation with a separate assembly for each. Horizontal isolation is provided by a pendulum assembly which supports a pneumatic vertical isolator (a piston, rolling diaphragm, and a two-chamber air tank). Sometimes these isolators use viscous fluid to damp the horizontal motion of the air chamber (which is connected to the payload by shearing the rolling rubber diaphragm). Such isolators could have a secondary fluid bath for damping the vertical motion as well, where the `bob` of the current invention could be fastened to the vertical motion piston assembly, achieving the same end.